Plasma processing is a common procedure that is widely used in semiconductor manufacturing to modify the surface of a semiconductor material. For example, plasma processing is widely used for etching and depositing materials onto the surface of a semiconductor substrate or “wafer.” Typically, the plasma is generated by a high-voltage discharge mechanism, and the ions created by this process are highly reactive. The ions can either etch the substrate, as in fluorine ions etching a silicon surface, or they can deposit material, as in metallic ions depositing metal onto the surface.
A deleterious side effect of plasma processing is the damage to the substrate surface created when the high-energy ions impact the surface. Because the plasma is typically within a high-voltage electric field, its ions are accelerated toward the substrate and strike the surface with substantial energy. These highly energetic ions are directional (typically, normal to the surface of the wafer) and can cause localized damage to the surface. This damage often results in semiconductor device performance degradation.
One way to reduce the impact of these energetic ions is to raise the voltage of the semiconductor substrate. This has the effect of reducing the velocity of the incident ions as they impact the surface of the wafer. Unfortunately, it also reduces the directionality, and hence often results in anisotropic etching of or depositing material onto the surface.
Another way to reduce the impact of the energetic ions of a plasma is to extract them from the plasma and allow them to flow “downstream” from the discharge in a neutral electric field onto the surface of the wafer. While this process significantly reduces the localized damage to the wafer, the etching and deposition rates are significantly reduced because the density of ions within the downstream neutral field is generally quite low.
There is also another issue with the plasma processing of wafers that relates to wafer throughput, which is the number of wafers per hour that can be processed by a given tool or in a given sequence of tools in forming a semiconductor device. In any semiconductor process, it is desirable to conduct that process as fast as possible and to move the wafer to the next process as fast as possible. It is desirable to reduce the amount of time a wafer spends being processed so that the wafer throughput for the particular tool or semiconductor manufacturing process can be maximized. In the case of plasma processing, it is time consuming to plasma process a wafer in a plasma processing system and then move the wafer out of the plasma processing system to the next system to carry out the next process.